1. Field of the Invention
The present invention relates to the field of drinking water treatment and, more specifically, to the removal of ammonia from drinking water.
2. The Prior Art
Ammonia occurs naturally in some groundwater, or is added to water to form chloramines in drinking water distribution systems. Research on the presence of ammonia in drinking water distribution systems has suggested some correlation between excess ammonia and increased biological activity (Servais et al. 1995; Wilezak et al. 1996) and adverse effects on water's taste and odor (Bouwer and Crowe 1988; Rittmann and Huck 1989).
Chloramines (a combination of ammonia and chlorine) are commonly used as disinfectants in the water treatment process in place of free chlorine because they produce lower levels of disinfection by-products such as trihalomethanes, haloacetic acids, and other halogenated organic compounds, which are potentially carcinogenic or mutagenic. Free ammonia is produced when chloramines break down within the distribution system (Vikesland et al. 2001) and when excess ammonia relative to chlorine is added in the chloramine production process.
The oxidation of ammonia to nitrite, and then nitrate, is a biological process referred to as nitrification. When nitrification occurs uncontrolled in drinking water distribution systems, the biological stability of the distribution system is disrupted, which can cause a number of water quality problems (Rittmann and Snoeyink 1984). The autorophic bacteria responsible for nitrification are abundant in many source waters and can grow readily in distribution systems if ammonia and oxygen substrates are available. The occurrence of nitrification in distribution systems is common and has been will-documented (Rittmann and Snoeyink 1984; Odell et al. 1996; Fleming et al. 2005). The growth of nitrifying bacteria in distribution systems can cause a number of problems. For example, biological activity has been shown to promote corrosion of some metals (Bremer and Wells 2001; Lee et al. 1980). In the case of nitrifying bacteria, the corresponding pH drop associated with the biological oxidation of ammonia directly impacts corrosion of distribution system materials.
In addition, the nitrifying bacteria support the development of undesirable heterotrophic bio-films by supplying organic carbon substrates. These biofilms produce metabolic byproducts that adversely affect the taste and odor of the water (Suffet et al. 1996). Incomplete nitrification of ammonia can result in increased levels of toxic nitrite (NO2). Because the United States Environmental Protection Agency's (U.S. EPA) maximum contaminant levels (MCLs) apply at the entry point into the distribution system, rather than within the distribution system, monitoring of contaminants such as nitrite and nitrate are normally not conducted at the consumer's tap. If nitrification resulting from elevated ammonia levels in the source water occurs in the distribution system, elevated and potentially dangerous levels of nitrite and nitrate can form and go unnoticed.
Many regions in the United States have excessive levels of ammonia in their source waters. For example, farming and agriculture in the Midwest contribute to relatively high levels of ammonia in many ground waters. Although ammonia in water does not pose a direct health concern, nitrification of significant amounts of excessive ammonia may. In addition, ammonia in arsenic bearing waters, for example, may negatively impact arsenic removal by creating a chlorine demand and reducing the chlorine's availability for oxidation of arsenic. Clearly, the complete oxidation of excess source water ammonia during the treatment process reduces the potential negative impact (nitrification) on distribution system water quality. While physicochemical methods for ammonia removal, such as ion exchange, are able to remove ammonia to varying degrees, biological approaches may be the most efficient and cost-effective.
Biologically-active filtration has been used successfully for addressing reduction of some contaminants in Europe for years. Bouwer and Crowe (1988) documented the use of various biological methods throughout Great Britain, France, and Germany, including fluidized beds, rapid sand filters, biologically active granulated active carbon (GAC), and soil-aquifer treatment. However, the use of biologically active filtration to oxidize ammonia as a full-scale drinking water treatment process has not been adopted in the United States because number of concerns including the potential release of excessive numbers of bacteria into finished waters, sensitivity of bacteria to changes in water chemistry and operating conditions, and a lack of long term documentation of the effectiveness and reliability of biological water treatment processes. Biological oxidation of ammonia requires oxygen to convert ammonia to nitrate. Many waters contain ammonia levels requiring greater levels of oxygen than can be introduced through accepted processes. The result is incomplete oxidation of ammonia and elevated levels of nitrite. Nitrite is more toxic than nitrate to humans and subsequently is regulated at a lower level (1 mg N/L) than nitrate (10 mg N/L), “N/L” meaning “as nitrogen, per liter.”